Such a telecommunication network node is already known in the art, for instance from the book "Transmission networking: SONET and the Synchronous Digital Hierarchy" by M. Sexton et al., Artech House, 1992 and more particularly from section 7.3.1 "Adaptation between Section and HO Path Layers" thereof. The known node, depicted in detail in FIGS. 7.26 and 7.27, handles input and output datastreams according to the Synchronous Digital Hierarchy SDH.
The node discussed in the above section is chosen so as to highlight a basic requirement for a network node of the above type, namely the fact that after dismantling of the input datastream its payload, represented by the successive data units, needs to be realigned in an output frame format synchronous with the transmit clock signal.
In the above known node this requirement is met by allowing the buffer means to absorb the effects of pointer justifications in the input datastream and of differences in phase and/or frequency between the transmit and receive clock signals. By reading out the buffer means at a rate varying as a consequence of the above effects these may be accounted for in the output datastream through pointer justifications. The known network node determines the required varying rate as a function of the buffer means filling level represented by the difference between the values of read and write pointers to this buffer. The output interface thus checks at every pointer justification opportunity if the buffer filling level is too high or too low and if so temporarily increases or decreases the rate at which it reads the buffer means in order to make a positive or negative pointer justification respectively.
A drawback of this known network node is that the filling level of the buffer means is not always a good measure of the effects which are to be translated in the above-mentioned varying rate. Indeed, if the successive data units experience a variable delay before being written in the buffer means, the latter variable delay accounts for most of the fluctuations in the buffer filling level and hence gives rise to undesired pointer justifications independent of the above mentioned effects. Also, if this variable delay is such that the successive data units do not arrive in sequence at the buffer means, it is not only hard to compute the filling level of the buffer means but it is also unrealistic to use such a filling level to determine the varying rate.